1. Field of the Invention
The present invention pertains to semiconductor processing, and in particular to a method of improving the temperature uniformity when photon-annealing a semiconductor substrate.
2. Description of the Prior art
In order to fabricate high-performance semiconductor devices such as integrated circuits (ICs), the manufacturing process typically includes an annealing process. The annealing process serves to activate dopants in the semiconductor wafer along the way to forming the final devices (e.g., transistors). One annealing technique involves irradiating the semiconductor wafer with short, intense pulses of radiation. This is referred to herein as “photo-annealing.” The short duration of the annealing radiation pulses allows for the fabrication of transistors with very low sheet resistance and ultra-shallow junctions, which translates into optimal device performance.
When lasers are used to perform the annealing, the photo-annealing process is referred to as “laser thermal processing” (LTP), or alternatively “laser thermal annealing” (LTA). One method of LTA applied to semiconductor manufacturing involves using a short-pulsed laser to thermally anneal the source and drain of the transistor to activate the implanted dopants therein. Under the appropriate conditions, i.e. melting of the junction material followed by rapid solidification, it is possible to produce source and drain junctions with activated dopant levels that are above the solid solubility limit. This produces transistors with greater speeds and higher drive currents. This LTP technique is disclosed in U.S. Pat. No. 5,956,603, which patent is incorporated by reference herein.
Other techniques for photo-annealing semiconductor substrates involve the use of flash lamps to create the pulse of radiation. For example, U.S. Pat. No. 4,151,008, entitled, “Method Involving Pulsed Light Processing of Semiconductor Devices,” discloses a method in which a pulsed laser or flash lamp produces a short duration pulse of light for thermal processing of selected regions of a semiconductor device. The light pulse is directed towards the semiconductor device and irradiates selected surface regions of the device to be processed. Energy from the absorbed light pulse momentarily elevates the temperature of the selected regions above threshold processing temperatures for rapid, effective annealing, sintering or other thermal processing. The characteristics of the light pulse are such that only those surface regions in the immediate vicinity of the flash, i.e. at the top of the substrate and within 100 microns of the edge of the field, are elevated to a high temperature. The remaining mass of the semiconductor device is not subjected to unnecessary or undesirable high temperature exposure and serves to quickly cool the heated area.
However, a shortcoming of this technique is that the dopant concentration cannot go beyond the solid solubility limit. Also, the technique suffers from temperature non-uniformities due to the different reflectivity properties of the transistor and related structures formed in and on the semiconductor substrate, prior to the annealing step.
Modern ICs contain a variety of device geometries and materials, and the accompanying structures formed in the semiconductor wafer have different reflectivities. To achieve uniform performance in each device, it is necessary that all devices be heated (annealed) to essentially the same temperature. This constrains the permissible variation in reflectivity of each device (e.g., transistor) in the circuit and constrains the spatial variations in the density of devices throughout the circuit. High-frequency spatial variations in reflectivity tend to be smoothed out by thermal conduction, but low frequency variations create similar variations in annealing temperature. For certain semiconductor processes, such as the process for activating dopants without melting the substrate (i.e., a non-melt process), the temperature of the surface of the semiconductor substrate must be kept very uniform (<±10° C.) and remain below the melting point of silicon (assuming the junction is not amorphized).
U.S. Pat. No. 6,635,588 (the '588 Patent) and U.S. Pat. No. 6,495,390 (the '390 Patent) describe respective techniques for controlling the amount of heat transferred to a process region of a workpiece (wafer) from exposure with a pulse of radiation. The '588 patent uses a thermally induced phase switch layer, and the '390 patent uses a thermally induced reflectivity layer. The phase switch layer is a film stack that includes an absorber layer and a layer of material that undergoes a phase transition. The absorber and phase transition layers are deposited atop a silicon wafer. The absorber layer absorbs radiation and converts the absorbed radiation into heat.
The phase transition layer can be deposited above or below the absorber layer and is chosen to have a phase transition temperature slightly above the desired maximum temperature of the underlying layers. The phase switch layer may include other layers, such as a thermal insulator layer and a layer to simplify stripping the phase switch layer once its function is completed. In some circumstances it is possible to combine some of the layers into a single layer serving multiple functions. The close proximity, of the phase switch layer and the device process region ensures that their temperatures are always very close. The high latent heat of the phase change holds the temperature of the phase switch layer constant once the phase transition temperature (TP) is reached. Provided the radiation dose has reasonable limits so that some, but not all, of the phase transition material is converted to the second phase by the radiation pulse, then tight control of the maximum temperature seen by the process region may be expected.
The reflectivity switch layer of the '390 patent operates in a similar manner, except that the reflectivity layer transitions from a non-reflective state to a reflective state at a fixed reflectivity transition temperature. The reflective state strongly inhibits the further absorption of heat by the switch layer and consequently, the process region.
While the inventions of the '588 Patent and the '390 Patent are effective in controlling the maximum temperature seen by the process region during LTP, each requires the use of an absorbing layer and an active layer that undergoes a transition at some temperature. For the melt annealing process, there are material combinations that work very well. However, for non-melt annealing, there are few if any desirable materials for the phase transition layer and it is highly desirable to minimize the number of layers that have to be added and removed from the substrate for the annealing step.
Accordingly, it would be desirable to have simpler, more cost effective, methods of controlling the amount of heat delivered to a semiconductor wafer and to improve temperature uniformity during photo-annealing. In particular, it would be desirable to reduce the number of layers and restrict the layers to materials commonly employed in silicon semiconductor fabrication.